1. Field of the Invention
This invention relates generally to megasonic processing apparatus and associated methods involving two or more piezoelectric transducers operating in thickness mode at different megasonic frequencies of at least 300 KHz or higher, and relates more particularly to improving performance by matching the fundamental resonant or higher-level harmonic frequencies of multifrequency transducers to the fundamental resonant or higher-level harmonic frequencies of a plate or other mounting structure upon which the transducers are mounted.
2. Description of the Relevant Art
Megasonic processing involves generating and using high frequency energy at frequencies above 300 KHz. Many megasonic systems operate at frequencies at or near 1,000 KHz (1 MHz). Although 1 MHz is the consensus, preferred frequency for many applications, the frequency range goes much higher, with frequencies as high as 10 MHz. Typical uses for megasonic systems include cleaning delicate objects, such as semiconductor wafers and disc drive media. Such a megasonic cleaning process involves placing the objects to be cleaned in a fluid-filled tank, and applying vibrational energy at megasonic frequencies to a radiating surface or surfaces of the tank. One or more piezoelectric transducers (PZT) are used to generate the vibrational energy. The PZTs are bonded or otherwise attached to a plate or tank but without external pressure or compression. A generator supplies an alternating current driving signal to the transducers. Megasonic transducers operate in thickness mode, where a piezoelectric element is excited by an alternating current driving signal that causes alternating expansion and contraction of the transducer, primarily expanding and contracting the thickness of the transducer. A piezoelectric transducer having a thickness of 0.080 inches has a fundamental, thickness mode, resonant frequency of 1,000 KHz. A thinner PZT will have a higher resonant frequency in thickness mode, and a thicker PZT will have a lower resonant frequency in thickness mode. It is known to sweep the driving frequency throughout a range of frequencies, as disclosed in my U.S. Pat. No. 7,598,654, to improve distribution and uniformity of the high frequency energy.
Megasonic processing happens at much higher frequencies than ultrasonic processing, which involves fundamental frequencies typically in the range of 25 KHz to 40 KHz and upper-level harmonics upwards of about 300 KHz. Ultrasonic transducers are typically mass-balanced, with inert masses on either side of a piezoelectric element, and have a significant radial component of movement at right angles to the thickness. The PZTs are circular rings with the centers cut out, and their frequency is determined by the relationship of the inside diameter to the outside diameter. One common construction of an ultrasonic transducer is to stack one or more ring-shaped piezoelectric elements between two masses, and to hold the assembly together with an axial compression bolt. Compressing a radial PZT increases its vibration amplitude, which is the reverse of what happens with a thickness mode PZT. Ultrasonic cleaning is based on cavitation, which is the formation and collapse of bubbles in the fluid.
At the frequencies used for megasonic cleaning, significant cavitation does not occur, so the cleaning action is based on another mechanism known as micro-streaming, which is a general flow of detached particles flowing away from the megasonic transducers. This flow consists of planar waves originating at the surface to which the transducers are mounted. If the transducers are mounted on the bottom of the tank, then the planar waves move upward in a vertical direction. The planar nature of these micro-streams affects the distribution of megasonic energy throughout the tank. There is little or no activity opposite empty spots or gaps between the megasonic transducers. One way to maximize the distribution is to cover a high percentage (e.g., 80-90%) of the surface area of the tank with transducers. Another but less efficient way is to oscillate or move the parts to be processed throughout the tank so that all surfaces are exposed to sufficiently high megasonic energy.
It is known to use multiple transducers of different frequencies in an ultrasonic apparatus, as shown for example in U.S. Pat. No. 6,019,852. This patent discloses mounting lower frequency transducers on a thicker plate than higher frequency transducers to counter excessive erosion caused by the lower frequency transducers.